Need help with a perhaps unique interview situation.

Here is the situation. I work in a hospital business/patient accounts office. There is an opening for a more advanced job within the same office. It would be moving from an accounts receivable position to a more IT related position. I would be reporting to my bosses boss(and interviewing with her). We are acquainted as we work in the same building. The dress code for men in my office is a shirt with a collar, slacks(jeans on Friday), and closed toe shoes(tennis shoes are acceptable).

The job is only open to people in the building.

Just curious if anybody thinks something less than a full on suit would be acceptable for an interview? All I have for a suit is a slightly ill fitting JC Penny special that is 10 years old. I don't have a lot of coin to drop on a new suit. On hand I have black dress shoes, light brown casual Merrell Lotus shoes, black, dark grey, and brown pants and various shirts and ties. Should I pick up a v-neck sweater to go with it or a sports coat or blazer? Thoughts?